Duplicate rows in MySQL db - php

Question

For some reason every once in a while my code will enter data into the database twice. I have changed my code and made some tweaks but have not had any luck. I know I must be over looking something I just don't know what.
<?php
$con = new mysqli("localhost", "******", "******", "******");
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$conpo = new mysqli("localhost", "******", "******", "******");
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$resultpo = mysqli_query($conpo, "SELECT * FROM fpo ORDER BY ponum DESC LIMIT 1");
while ($rowp = mysqli_fetch_array($resultpo)) {
$po = $rowp['ponum'];
$ponum = ++$po;
}
$date = $_POST['date'];
$to = $_POST['to'];
$time = $_POST['time'];
$vin = $_POST['stk_vin'];
$reason = $_POST['reason'];
$amount = $_POST['amount'];
$empnum = $_POST['emp'];
$mananger = $_POST['mananger'];
$accnum = $_POST['acc'];
$store_location = $_POST['store'];
$borr = $_POST['borrowed'];
$oldid = $_POST['oldid'];
mysqli_query($con,
"INSERT INTO fpo (" .
"`ponum`, `date`, `to`, `time`, `vin`, `reason`, `amount`, `empnum`, `mananger`, `accnum`, `store_location`, `borr`" .
") VALUES (" .
"'$ponum', '$date', '$to', '$time', '$vin', '$reason', '$amount', '$empnum', '$mananger', '$accnum', '$store_location', '$borr'" .
")");
?>
Here is the html forum page.
<form action="print.php" method="POST" name="form1" id="form1">
<table width="800" border="0" align="center" cellpadding="0" cellspacing="0" class="table" style="width:700px">
<tbody>
<tr>
<td height="19" colspan="2" valign="middle" class="to" style="width:450px" ><div class="logo"></div></td>
<td width="95" rowspan="2" valign="bottom"><div class="date">Date:
<label>
<input name="date" type="text" readonly id="date" value="07/15/2015" size="10" />
</label>
</div></td>
<td width="106" rowspan="2" valign="bottom" class="po" style="width:100px" ><div class="date">PO: <span id="sprytextfield3">
<label>
<input name="po" readonly type="text" value="14739" />
<input type="hidden" name="oldid" value="11810">
</label>
<span class="textfieldRequiredMsg">A value is required.</span></span></div></td>
</tr>
<tr>
<td height="20" colspan="2" valign="middle" class="to" style="width:450px" ><div class="toheader">To: <span id="spryselect1">
<label>
<select name="to" id="to">
<option selected="selected" disabled="disabled">Select One</option>
<option value="1">Kum-N-Go</option>
</select>
</label>
<span class="selectRequiredMsg">Please select an item.</span></span>
<input name="time" type="hidden" id="time" value="
11:13:33 ">
</div></td>
</tr>
<tr>
<td colspan="2" style="height:125px" ><b> Reason For Gas Ticket: </b> <br>
<span id="spryselect2">
<label>
<select name="reason" id="reason">
<option selected="selected" disabled="disabled">Select One</option>
<option value="Advertising">Advertising</option>
<option value="Auction">Auction</option>
</select>
</label>
<span class="selectRequiredMsg">Required</span></span><br>
<br>
Car Borrowed From Another Dealership? <br>
<span id="spryselect3">
<label>
<select name="borrowed" id="borrowed">
<option selected="selected" disabled="disabled">Select One</option>
<option value="Yes">Yes</option>
<option value="No">No</option>
</select>
</label>
<span class="selectRequiredMsg">Please select an item.</span></span><br></td>
<td><b>stk/vin/RO#:</b><br>
<span id="sprytextfield2">
<label>
<input name="stk_vin" type="text" id="stk_vin" onclick="return stk()" size="12" />
</label>
<span class="textfieldRequiredMsg"><br />
Stock or Vin .</span></span></td>
<td align="center"><b> Amount<br>
<div class="money"><span id="spryselect5">
<label><b>
<select name="amount" id="amount">
<option value="5.00" selected="selected">5.00</option>
<option value="1.00">1.00</option>
<option value="2.00">2.00</option>
<option value="3.00">3.00</option>
<option value="4.00">4.00</option>
<option value="5.00">5.00</option>
<option value="6.00">6.00</option>
<option value="7.00">7.00</option>
<option value="8.00">8.00</option>
<option value="9.00">9.00</option>
<option value="10.00">10.00</option>
</select>
</b></label>
<span class="selectRequiredMsg">Please select an item.</span></span></div>
</b></td>
</tr>
<tr>
<td width="257">Employee Requesting Po:<span id="spryselect9">
<label>
<select name="emp" id="acc">
<option selected="selected" disabled="disabled">Select One</option>
<option value="5223">5223</option>
</optgroup>
</select>
</label>
<span class="textfieldRequiredMsg">Required</span><span class="textfieldInvalidFormatMsg">Required</span></span></td>
<td width="219">Acc #:<span id="spryselect4">
<label>
<select name="acc" id="acc">
<option selected="selected" disabled="disabled">Select One</option>
<option value="51">Sales</option>
<option value="52">Services</option>
<option value="53">Parts</option>
<option value="FI">F&I</option>
</select>
</label>
<span class="selectRequiredMsg">Required</span></span></td>
<td colspan="2" align="right"><label> Mananger:
<input name="mananger" onclick="return getfoc()" type="text" id="mananger" size="12" readonly />
</label></td>
</tr>
<tr>
<td></td>
<td>Store: <span id="spryselect6">
<label>
<select name="store" id="store">
<option disabled="disabled" selected="selected">Select One</option>
<option value="chevy">Chevy</option>
<option value="Ford">Ford</option>
<option value="Dodge">Dodge</option>
</select>
</label>
<span class="selectRequiredMsg">Required</span></span></td>
<td colspan="2" align="right"><div id="btn"> </div>
<script language='javascript' type='text/javascript'>
function GetValueFromChild(myVal)
{
document.getElementById('mananger').value = myVal;
document.getElementById('btn').innerHTML = '<input type="submit" id="register" value="Print Ticket" />';
}
</script></td>
</tr>
</tbody>
</table>
<p> </p>
</form>

Answer 1

Because you are not doing atomic operation. Lets assume the below flow:
SELECT1 -> SELECT2-> INSERT1-> INSERT2
How are avoid this - no synchronization or locking. You can avoid this with few options.
Use a lock before entering this block and free once insert is done( in PHP). This can be based on file lock or memcache lock
You can add an index on the column you want to keep unique say, (ponum, from, to). Instead of using INSERT, you can use INSERT IGNORE

Answer 2

I am pretty sure your users are clicking the submit button twice I had this issue before I fixed it using jquery's validator such as following.
$(document).ready(function(){
$("#form1").validate({submitHandler: function(form) {if (!this.beenSubmitted) {this.beenSubmitted = true;form.submit();} }});
});